Theoretical Pathway toward Improved Reverse Osmosis Membrane Selectivity for Neutral Solutes: Inspiration from Gas Separations

Highly
rigid membrane materials with tailored structures
have exhibited
permeabilities and selectivities that exceed the polymer upper bound
in gaseous and organic solvent separations for challenging mixtures
containing species that are similar in size and shape. One potential
question is whether such membrane materials can maintain meaningful
guest diffusivities in situations where the microporous spaces are
essentially full of guest molecules. Here, we use a simplified transition
state theory approach to estimate the diffusivity of water and small
organics within a microporous membrane. The transition state theory
model is parameterized using experimental values from zeolites and
carbon molecular sieve materials found in the literature. We demonstrate
the differences in transport and Maxwell–Stefan diffusivities
based on guest species loading with different isotherm behaviors.
These calculations theorize a path forward for highly selective reverse
osmosis membranes for aqueous phase separations.